The shifting of these massive casks holding used nuclear fuel was the first caused by an earthquake in the U.S., according to the U.S. Nuclear Regulatory Commission.

None of the metal cylinders was damaged and no radiation was released, Dominion Virginia Power said.

Like hockey pucks on a jostled tray, the 16-foot tall casks shifted from an inch to 41/2 inches, utility company spokesman Rick Zuercher said.

"They just moved because of the vibration," Zuercher said. "They remained upright and fully intact."

Similar storage casks at Dominion's Surry nuclear power plant were not affected, Zuercher said.

"There was a very minor tremor there and there was no affect on storage or operation," he said.

Federal regulators will examine data from the spent-fuel storage area as part of the inspection of the North Anna plant that the NRC began Tuesday, said Scott Burnell, a spokesman with the federal agency's headquarters in Rockville, Md.

"The information available indicates the shifting did not affect safety in any way," Burnell said. "It is an instance of an event we had not previously seen, so were trying to learn as much as possible."

Used nuclear fuel rods from commercial atomic power plants are highly radioactive.

Dominion Virginia Power has 27 of the massive TN-32 storage casks standing vertically outdoors on the concrete pad. The casks, made from thick steel, are not fastened to the ground.

Concrete bunkers for other used-fuel containers stored horizontally at the Louisa County power station experienced "cosmetic" damage, he said. "None of these moved."

NRC regulations specify the spacing of the vertically set dry casks from one another mainly to ensure workers are not exposed to unexpected levels of radiation, Burnell said.

"If the spacing shifts a slight amount, the dose shifts a very slight amount," he said. "Those doses have to be taken into account. The doses would be very low in any case."

The NRC requires a nuclear plant's assemblies of spent fuel rods to be cooled in water pools for at least five years before being transferred to dry casks.

Spent-fuel storage in casks is considered safe, the NRC said. Such storage systems are designed to resist floods, tornadoes, projectiles, temperature extremes and other unusual scenarios, the federal agency said.

During the past 20 years, the NRC said, there have been no radiation releases affecting the public, no radioactive contamination, and no attempts to sabotage spent-fuel casks or their storage sites across the U.S.

Last week's magnitude-5.8 earthquake shook protective electrical devices at the North Anna Power Station strongly enough to cause the plant to shut down automatically, the first time this has occurred in the United States.